Higher plants are able to produce a large number of small-molecular-weight compounds with very complex structures. These compounds, called “secondary metabolites”, can play a role in the resistance against pests and diseases, attraction of pollinators and interaction with symbiotic microorganisms. Besides the importance for the plant itself, secondary metabolites are of great commercial interest because they determine the quality of food (color, taste, and aroma) and ornamental plants (flower color, smell). A number of secondary metabolites isolated from plants are commercially available as fine chemicals, for example, drugs, dyes, flavours, fragrances and even pesticides. In addition, various health improving effects and disease preventing activities of secondary metabolites have been discovered, such as anti-oxidative and anti-metastatic-lowering properties (e.g., vinblastine, taxol).
Although about 100,000 plant secondary metabolites are already known, only a small percentage of all plants have been studied to the extent necessary for the determination of the presence of secondary metabolites. It is expected that interest in such metabolites will continue to grow as for example, plant sources of new and useful drugs are discovered. Some of these valuable phytochemicals are quite expensive because they are only produced at extremely low levels in plants.
Very little is known about the biosynthesis of secondary metabolites in plants. However, some recently elucidated biosynthetic pathways of secondary metabolites are long and complicated requiring multiple enzymatic steps to produce the desired end product. Most often, the alternative of producing these secondary metabolites through chemical synthesis is complicated due to a large number of asymmetric carbons and in most cases chemical synthesis is not economically feasible.
The recovery of valuable secondary metabolites is mostly achieved through extraction and purification (generally at low yields) of imported, sometimes exotic, plant biomasses, whose reproductive agriculture and secure long term supply are often very difficult, if not impossible to guarantee. The problems of obtaining useful metabolites from natural sources may potentially be circumvented by cell culture. The culture of plant cells has been explored since the 1960's as a viable alternative for the production of complex phytochemicals of industrial interest. Although plant cell cultures might be somewhat sensitive for shear forces, many cultures can be grown in large bioreactors without difficulty. For example, the use of large-scale plant cell cultures in bioreactors for the production of alkaloids has been extensively studied (Verpoorte et al. (1999) Biotechnol. Lett. 21, 467). Since it has been observed that undifferentiated cultures such as callus and cell suspension cultures produce only very low levels of secondary metabolites one tends to use differentiated plant cell cultures such as root- and hairy root-culture. For example, tropane alkaloids that are only scarcely synthesized in undifferentiated cells are produced at relatively high levels in cultured roots.
Despite the promising features and developments, the production of plant-derived pharmaceuticals by plant cell cultures has not been fully commercially exploited. The main reasons for this reluctance shown by industry to produce secondary metabolites by means of cell cultures, compared to the conventional extraction of whole plant material, are economical ones based on the slow growth and the low production levels of secondary metabolites by such plant cell cultures. Important causes are the toxicity of such compounds to the plant cell, and the role of catabolism of the secondary metabolites. Another important problem is that secondary metabolites are mostly retained intracellularly complicating the downstream processing and purification. Indeed, often laborious extraction schemes have to be developed for each specific secondary metabolite of interest.